How does one overcome trust and insecurity issues when in a relationship?
First of all, I'm so glad that you reached out, it's not always easy to ask for advice, especially when it comes to those intimate relationships!
I think what you're going through is not uncommon but it is important to recognize and change patterns so that people feel better supported and we avoid following the same patterns over and over again. I'm proud of you for recognizing that perhaps your thinking patterns are not accurate and that it may be you jumping to conclusions rather then putting it on the other person completely, but let's take a look on what you are experiencing.
I think we need to start by taking a look at you and your partner and the reasons that you have those thoughts that perhaps they are cheating on you or doing something unfaithful. Has this happened in the past? I know that you referenced your childhood, was there any instability that could have led you to think that a partner is not capable of being faithful to you? While we don't necessarily follow all the patterns that we see as children, what we grew up with as "normal" does have an effect on how we perceive our lives and what is "acceptable" or what we believe will happen in the future. Do you feel like that's acceptable? Have you settled for that in the past?
I think it's important to look at the values that you are seeking in a partner and if your current partner has those, giving them the benefit of the doubt and talking to them about your fears openly and finding ways that they can reassure you without feeling like you are accusing them of being disloyal. That communication may look like "I don't think that you are cheating on me, but I've struggled in every relationship with trust and I just need a little extra reassurance sometimes". If your partner is committed and understanding, they will understand that you're not accusing them and that it comes from past responses. If you get a feeling that something is wrong however, it's also good to listen to your instincts and recognize that you don't need to settle for someone who makes you feel uncertain.
It sounds like you've had a lot going on in your life and that it's hard to let your guard down. At some point we have to choose to trust, and that's not easy. Just know that not everyone is out to get you and that there are still wonderful people in the world!
I hope this helps and I wish you all the best in the future.